Omni-directional long slot antenna



.. D W. DARLlNG Owl-DIRECTIONAL LONG SLOT ANTENNA 2 Sheets-Sheet lINVENTOR Woodrow Darling BY ZdA %M Filed March 2, 1956 h lm w it UnitedStates Woodrow Darling, Merchantville, N. J., assignor to RadioCorporation of America, a corporation of Delaware Application March 2,1956, Serial No. 569,134 Claims. (Cl. 343-770 This invention relates toantennas of the type wherein the radiating elements are slots in aconductive surface, and particularly to vertically arranged broadcasttype antennas having an omni-directional characteristic in thehorizontal plane and a directive characteristic in vertical planes, tobest serve home receivers located at various distancesaround theantenna. The invention is particularly useful as a televisionbroadcasting antenna in the very high frequency range from 76 to 216megacycles.

It is an object of this invention to provide an improved omi-directionalslot antenna having a longer radiating slot associated with a singlefeedpoint than has heretofore been practical, with the result thatimportant economies in the feed system are realized.

It is another object to provide an improved simple omni-directionalantenna which may be mounted on one side of a supporting tower withoutappreciably affecting the omni-directionalcharacteristics of theantenna;

It is a further object to provide an improved antenna system forradiating visual and aural television signals from separate antennas ona single tower so arranged as to eliminate the needfor a diplexer tocombine the out puts of the respective transmitters.

It is a still further object to provide an improved omnidirectionalantenna having a basic structure which can easily be modified duringassembly for use at any desired frequency band within a large range offrequencies.

An antenna according to the invention includes an elongated conductivemember having a longitudinal slot therein, the conductive member beinginterrupted at points equidistant from the longitudinal center of theslot and provided with crossover members bridging the interruptions. Bythis arrangement, the slot may be made to have an electrical lengthequal to a plurality of half wavelengths in the slot at the operatingfrequency. A feeder line is connected across the slot at thelongitudinalcenterpoint. Additional conductive members are provided'to cooperatewith said elongated conductive member to define an elongated cavity incommunication with the slot throughout its length. The cross-sectionaldimensions of the elongated cavity are varied along the length of thecavity in such a way that the plurality of half wave portions of theslot are substantially equally energized from the single feeder line.Stated another way, the impedance of the elongated cavity issymmetrically varied from the center towards the two ends in such a wayas to uniformly energize the plurality of half wavelength sections ofthe slot.

For a better understanding of the invention, reference is had to thefollowing more detailed description taken in conjunction with theappended drawings wherein:

Figure 1 is a side view of an antenna constructed according to theteachings of this invention; v

Figure 2 is a front view of the antenna of Figure 1;

Figure 3 is a perspective view showing the top half of the antenna ofFigures 1 and 2 in enlarged form to reveal constructional details;

Patented Sept. 17, 1957' ice Figure 4 is an outline diagram illustratingtwo antennas of the type shown in Figures 1 and 2 mounted on a singletriangular tower, one of the antennas being connectedto an auraltransmitter, and the other antenna being connected to a visualtransmitter;

Figure 5 is an outline diagram illustrating four antennas of the typeshown in Figures 1 and 2 mounted on a single triangular tower forradiating the visual and aural outputs of two independent televisionstations;

Figure 6 is an outline sectional or top view of the structures ofFigures 4 and 5 illustrating the antennas sup ported by the triangulartower; and

Figure 7 is a diagram illustrating the electrical boundaries of thecavity in the antenna, which is in communication with the radiatingslot, and by means of which the plurality of half wave sections of theslot are substantially uniformly energized from the single feedpoint.

Referring in greater detail to the drawings, the antenna shown inFigures 1 through 3 includes electrically conductive angle members 10,11 and 12 arranged in end-toend relationship beside similarly arrangedconductive angle members 13,- 14, and 15. A metallic shorting plate 16is connected across the top ends of angle members: 10 and 13; and ametallic shorting plate 17 is connected. across the bottom ends of anglemembers 12 and 15.. Metallic crossover members 18 and 19 connect thebottom end of angle member 13 to the top end of angle member 11, and thebottom end of angle 10 to the top end of angle 14. Similarly, metalliccrossover members 20 and 21 connect the bottom end of angle member 14 tothe top end of angle member 12 and the bottom end of angle member 11 tothe top end of angle member 15. The members thus far described may beviewed as constituting an elongated conductive front member having alongitudinal slottherein, the conductive member being interrupted ortransversely slotted at points equidistant from the longitudinal centerof the slot and the interruptions being bridged by crossover members.The slot extends longitudinally from the shorting plate 16 to theshorting plate 17 and has a center line 22 about which the structure issymmetrical.

A feeder transmission line including an outer conductor 23 and an innerconductor 24 is connected across the slot at the center. The connectionis made by connecting the outer conductor 23 to the angle member 11 andby connecting the inner conductor 24 to the angle member 14 through acapacitive plate 25. The capacitive plate 25 is dimensioned to balancethe effect of the outer conductor 23 on the opposite side of the slot.

The antenna also includes a structural metallic back member 27 and aplurality of channel-shaped metallic side members 28 connecting thestructural back member 27 to the angle members defining the slot at thefront of the antenna structure. The side members 28 are sufficientlyclosely spaced with reference to the wavelength at the operatingfrequency so that they constitute a conductive sheet. Sheet metal may,of course, be substituted for the individual side members 28 providedthat interruptions are maintained at the crossover points. Individualside members 28 are preferred to sheet members because they provide morestrength for a given weight of material, and they result in a structurehaving less wind resistance than a solid imperforate structure. Aplurality of parallel horizontal conductive bars 33 through 41 areconnected between certain of the side members 28 as shown in Figure 3illustrating the upper half of an antenna. The lower half of the antennais a mirror image of the upper half. The bars 33 through 41 aresufficiently closely spaced relative to the wavelength at the sheet.

The vertical angle members through 15, the conductive side members 28,the conductive bars 33 thru 41, and the central portion of thestructural back member 27 (at points designated 31 and 32 ins-Figure 3)define a cavity in communication with, theslot formed by the anglemembers 10 through and the. shorting plates 16, 17; The cavity and. theslot extend. a total distance corresponding with two nominal electricalwavelengths in the cavity. The cavity and the slot have a lengthcorresponding with about 4 wavelengths in free space. The cavity isrestricted in one cross-sectionaldimension, that is, depth, by meansofthe conductive bars 33 through 41. It will be noted that the conductivebars 33 through 41 are locatediat varying distances from the anglemembers 10'- through 15. The arrangement below thecenter line 2 2 of theantenna issymmetrical with that shown in Figure 3 above the center line.It is thus apparent that there is provided an elongated cavity which isin communication with the radiating slot along its entire length, andthat a cross-sectional dimension of the cavity is varied alongits lengthfrom the center towards both ends to same side of the antenna as isviewed in Figure 1. The

ppints 31 and 32 represent the back member 27, and the points 33 through41 represent the correspondingly numbered conductive bars in Figure 3 ofthe drawings. The lefthand vertical line represents the radiating slotextending from the shorting plate 16 to the shorting plate 17. Thepositions of the crossover members 18, 19 and 20, 21 are indicated.

Figure 7 illustrates how one cross-sectional dimension of the cavity,namely the depth, varies along the length of the cavity and theassociated radiating slot. The center portion 45 of the cavity has themaximum depth. The end portions 46 and 47 are of reduced depth. Thesections of the cavity labeled T may be viewed as impedance transformingportions for matching the. impedance of, the center portion 45 to theend portions 46 and 47. Or, the conductive bar 35 may be considered asproviding an impedance discontinuity which establishes the end portion46 as a resonant cavity, and wherein the conductive bars 33 through 35-constitute an impedance transforming portion for properly coupling thecavity 46 to the central cavity 45. The conductive bars constituteinductive impedanceelements and may be replaced by capacitive elementswhich similarly vary an effective cross-sectional dimension of thecavity. Of course, the arrangement below the center line 22 is a mirrorimage of that above the center line.

Since the center portion 45 of the cavity has a greater depth than theend portions 46 and 47, the velocity of propagation of radio frequencyenergy outwardly from the feedpoint is greater in the center portion 45than it is in the end portions 46 and 47 of the cavity. Or, it may bestated that the phase velocity is greater in the outer cavity than inthe inner cavity. Therefore, an electrical half wavelength at theradiating slot opposite the center portion 45 of the cavity has ashorter physical length than an electrical half wavelength along theslot in the end portions 46 and 47 of the cavity. The increased physicallength of the half wave slot sections opposite the end portions 46 and47 of the cavity is advantageous in that itextends the physical lengthof the radiating slot and thereby improves the directivity of theantenna in vertical planes. In other words, most of the radiated energyis directednormal to the vertically extending slot and little energy iswasted by being directed upwardly into the sky or downwardly into theearth.

The depth of the center portion 45 of the cavity at the center line 22is selected to provide an impedance which matches the characteristicimpedance of the feeder line 24, 25. The impedance of the cavity lookingupwardly from the center line 22 is in parallel with the impedance ofthe cavity looking downwardly from the center line 22. Therefore, thetotal impedance of the cavity presented to the feeder line is equal tohalf the impedance looking in one direction from the center line 22.

It will be observed from the diagram of Figure 7 that the radiating slotof the antenna has a nominal length of two electrical wavelengths. Allthe energy radiated from the slot is in phase because of theappropriately positioned crossover members 18, 19, and 20, 21. Thecharacteristics of the antenna are such as to provide a more directivepattern in vertical planes than can be obtained from four independentlyfed half wavelength radiators of equal phase and current amplitude.Additionally, the antenna of this invention is superior to known arraysof antenna elements for the reason that the antenna is energized from asingle feedpoint by a single feeder line. Feeder lines are normallyconstructed in coaxial line form and are relatively expensive. The usualarray of antenna elements must be fed from a feeder line harnessconstructed to include additional lengths of feeder line to insure thefeeding of each antenna element in the same phasev as the others. Theadditional lengths of feeder line are not only expensive but aredifficult to mount on the antenna structure because of spacerequirements and because of disturbing effects they may have on theradiation characteristics of the antenna. The center portion of thecavity may have a length of only one nominal half wavelength if desired,but this reduces the length of the radiating slot and the directivitycharacteristic in vertical planes.

Reference will now be made to Figures 4 through 6 for a description ofthe manner of mounting the antenna of Figures 1 through 3 on atriangular tower. A tower of triangular cross-section is advantageousfrom a structural point of view for the reason that it provides thenecessary mechanical strength with a minimum of material. Unlike someantennas, the antenna of this invention is suitable for mounting on atower of triangular cross-section without impairing its radiationcharacteristics.

Referring to Figures 4 and 6, the triangular tower is shown in outline.The tower 50, ofcourse, consists of vertical members and many crossbracing structural members. The outline 51 of one antenna like thatshown in Figures 1 through 3 is mounted at one corner of the triangulartower, and the outline of another similar antenna 52 is shown at anothercorner of the tower. The antenna 51 is connected by means of a feederline 53 to an aural transmitter, and the antenna 52 is connected bymeans of a feeder line 54 to a visual transmitter. The

aural antenna 51 and the visual antenna 52 are similarly constructed butare slightly differently dimensioned to accommodate the slightlydifferent frequencies of the aural and visual signals of a giventelevision channel. Both the aural signal and the visual signal areindependently radiated with omni-directional patterns in the horizontalplane and without interference between the two antennas. Thecross-coupling between the aural and visual antennas is found inpractice to be down about 26 decibels.

The arrangement shown in Figure 4 for radiating aural and visual signalsis very advantageous for the reason that the two antennas are connecteddirectly to the respective aural and visual transmitters without theneed for an intervening diplexer. Diplexers are necessary for some typesof antennas to combine the outputs of the aural and visual transmittersprior to application to antenna ele: ments which radiate both signals.Diplexers must handle high radio frequency powers and are thereforerelatively large and expensive.

Figure 5 shows an arrangement for transmitting thevisual and auralsignals from two television stations A and B. The visual antenna 53 andthe aural antenna 54 are associated with television station A, and thevisual antenna 55 and aural antenna 56 are associated with televisionstation B. The television stations A and .B are of course assigneddifferent frequencies corresponding with different television channels.The signal from one television station does not interfere with thesignal from the other television station. Since there are relatively fewlocations ideal for mounting an antenna to serve a community, and forobvious economic reasons, it is very advantageous to mount antennas fora plurality of television stations on a single tower.

An antenna according to the invention which was constructed and is inusefor transmitting a channel 13 television signal (210 to 216 megacycles)has the dimensions shown on Figures 1 through 3 of the drawings. Theconductive bars 36 through 41 are spaced 5% inches from the radiatingslot. Conductive bar 33 is spaced 6% inches, bar 34 is spaced 4 inches,and bar 35 is spaced 2% inches from the radiating slot. The feeder line23, 24 is a 3 inch diameter coaxial line having a characteristicimpedance of 51.5 ohms. The dimensions, of course, are given solely byway of example. The antenna described radiates 50 kilowatts oftelevision radio frequency signal.

It is apparent that according to this invention there is provided arelatively simple, economical and effective antenna wherein a relativelylong radiating slot is fed from a single feedpoint. The construction issuch that the antenna is easily modified during assembly to accommodateany one of many different frequency bands. The antenna provides anomni-directional pattern in the horizontal plane and is highly directivein vertical planes.

What is claimed is:

1. An antenna comprising, an elongated conductive member having alongitudinal slot therein, interruptions in said conductive member atpoints substantially equidistant from the longitudinal center of saidslot, crossover members bridging said interruptions, whereby said slothas a plurality of electrical half wavelength sections, a feeder lineconnected across said slot at substantially the longitudinal centerthereof, and additional conductive members cooperating with saidelongated conductive member to define an elongated cavity incommunication with said slot, said cavity having at least onecross-sectional dimension varying along the length of the cavity tosubstantially uniformly energize the plurality of electrical halfwavelength sections of said slot from said feeder line.

2. An antenna comprising, an elongated conductive member having alongitudinal slot therein, interruptions in said conductive member atpoints substantially equi distant from the longitudinal center of saidslot, crossover members bridging said interruptions, whereby said slotis a plurality of electrical half wavelengths long, a feeder lineconnected across said slot at substantially the longitudinal centerthereof, and additional conductive members cooperatnig with saidelongated conductive member to define an elongated cavity incommunication with said slot, said cavity having a center portion withcross-sectional dimensions to match the impedance of the cavity to theimpedance of the feeder line, said cavity having end portions of reducedcross-sectional dimension, and said cavity having impedance transformingportions of varying cross-sectional dimension between the center portionand the end portion of the cavity.

3. An antenna comprising, an elongated conductive member having alongitudinal slot therein, interruptions in said conductive member atpoints substantially equidistant from the longitudinal center of saidslot, crossover members bridging said interruptions, whereby said slothas a plurality of electrical half wavelength sections, a feeder lineconnected across said slot at substantially the longitudinal centerthereof, and additional conductive members cooperating with saidelongated conductive member to define an elongated cavity incommunication with said slot along the length thereof, said cavityhaving a depth from said slot which varies symmetrically from thelongitudinal center towards the ends to substantially uniformly energizethe plurality of electrical half wavelength sections of said slot fromsaid feeder line.

4. An antenna comprising, an elongated conductive member having alongitudinal slot therein, interruptions in said conductive member atpoints substantially equidistant from the longitudinal center of saidslot, crossover members bridging said interruptions, a feeder lineconnected across said slot at substantially the longitudinal centerthereof, and additional conductive members cooperating with saidelongated conductive member to define an elongated cavity incommunication with said slot along the length thereof, said slot andsaid elongated cavity being dimensioned to provide a nominal electricallength of two wavelengths therein, the entire slot being energized fromsaid single feeder line.

5. An antenna comprising, an elongated conductive member having alongitudinal. slot therein, interruptions in said conductive member atpoints substantially equidistant from the longitudinal center of saidslot, crossover members bridging said interruptions, a feeder lineconnected across said slot at substantially the longitudinal centerthereof, and additional conductive members cooperating with saidelongated conductive member to define an elongated cavity incommunication with said slot along the length thereof, said additionalconductive members being interrupted at points adjacent saidinterruptions in said elongated conductive member, and im pedance meansin said elongated cavity to energize said slot along its entire lengthfrom said single feeder line.

6. An antenna comprising, an elongated conductive structure having afront, two sides and a back, said front being provided with alongitudinally extending slot, said front and sides being interrupted attwo points equidistant from the longitudinal center of the slot todivide the structure into a central section having a nominal electricallength of one wavelength and two end sections each having a nominalelectrical length of a half wavelength, crossover members bridging theinterruptions in said front, a feeder line connected across said slot atthe longitudinal center thereof, two impedance discontinuities in saidstructure to divide the interior thereof into a central cavity and twoend cavities, the depth of the central cavity being varied symmetricallyfrom the center feedpoint to said impedance discontinuities to providean impedance transformation.

7. An antenna comprising, an elongated conductive structure having afront, two sides and a back, said front being provided with alongitudinally extending slot, said front and sides being interrupted attwo points equidistant from the longitudinal center of the slot todivide the structure into a central section having a nominal electricallength of one wavelength and two end sections each having a nominalelectrical length of a half wavelength, crossover members bridging theinterruptions in said front, a feeder line connected across said slot atsubstantially the longitudinal center thereof, two impedancediscontinuities in said structure to divide the interior thereof into acentral cavity and two end cavities, the depth of said end cavities fromthe front to the back being as shallow as practical to propagate thedesired radio frequency signal, whereby the physical length of the endsections is as great as practical, the depth of said center cavity atthe feedpoint being sufficiently great to provide an impedance match tosaid feeder line, the depth of said center cavity being variedsymmetrically from the center feedpoint to said impedancediscontinuities to provide an impedance transformation between thecenter cavity and the end cavities.

8. An antenna comprising an elongated conductive structure having afront, two sides and a back, said front being provided with alongitudinally extending slot, said front and sides being interruptedalong said structure at points spaced an integral number including oneof nominal electrical half wavelengths apart, and crossover membersbridging the interruptions in said front.

9. An antenna comprising an elongated conductive structure having afront, two sides and a back, said front being provided with alongitudinally extending'slot, said front. andsides being interrupted byatransverse slot intermediate, the ends thereof, and two conductivecrossover members having ends connected to said front and bridging saidtransverse slot.

10. Atransmitting antenna system comprising a tower having-Ha triangularcross-section, a pair of elongated antennas ofrectangular cross-sectionmounted on two corncrsof said tower, each of said antennas including anelorngated conductiyestructure having; a;front, two sides and;a -bagk;said front being provided with a longitudinal- 1y extending slot, saidfront and sides being interrupted along said structure at pointsspacedan integral number including oneof nominal electrical half wavelengthsapart, and crossoyer members bridging the.- interruptions in said front.

No references cited.

